The present invention relates to an all-position TIG (tungsten-inert gas) welding process.
Since excellent qualities of welds are obtained by TIG welding processes, the latter are widely used in welding various pipes in any welding position. In general, TIG welding processes may be divided into low-frequency pulse TIG welding processes which are most commonly practiced and high-frequency pulse TIG welding processes which are reserved for special purposes.
In the case of the low-frequency pulse TIG welding process, the welding current is controlled by the electric current pulses of low frequencies of a few hertz while the welding torch is moved. The weld zone is subjected to repeated melting and solidification and a bead is obtained. Therefore, the low-frequency pulse TIG welding process is best adapted in welding in any position. In addition, the low-frequency pulse TIG welding machines are simple in construction and are highly efficient in operation when used at field sites. However, the low-frequency pulse TIG welding has an inherent disadvantage in welding efficiency; that is, the metal deposition rate is low. For instance, in the case of a vertical downward welding position, which is most commonly practiced, the metal deposition rate is less than 8 g/min (with heat input of 7,500 Joule/cm). As a result, the number of passes must be increased so that the set-up time required for rewinding the welding cables and the hoses for inert gas is increased and subsequently the welding must be interrupted intermittently for a relatively long period. Thus, the productivity is low.
In the high-frequency pulse TIG welding processes, high-frequency pulse arcs of 2,000 to 25,000 Hz are used. In synchronism with the mechanical weaving of a welding torch, the output of the high-frequency current is controlled, whereby the pipes can be welded in all positions. Since an arc stream which is small in cross section and well stabilized can be obtained, satisfactory penetration can be achieved even when grooves have narrow widths, and a large molten metal pool can be maintained as compared with the low-frequency pulse TIG welding processes. However, the high-frequency pulse TIG welding processes have also a disadvantage in that when the distance between the electric power source and the welding torch exceeds a few meters, the high-frequency components are suddenly decreased due to inductance of the welding cable. As a result, the desired welding effects peculiar to the high-frequency pulse TIG welding processes cannot be attained at all under common or general welding set-ups. As a result, the efficiency of all position welding is adversely affected. For instance, in the case of the vertical downward welding, the metal deposition rate is on the order of 15 g/min (with the heat input of 15,000 Joule/cm).
The efficiency of the all position welding is greatly influenced by how well a molten metal pool is maintained. Basically, in order to improve the metal deposition rate and subsequently the welding efficiency, a large molten metal pool must be maintained. However, the volume of the molten metal pool is limited because of the force of gravity in the case of the all position welding. The molten metal pool is most difficult to be maintained especially in the vertical downward welding position. This will be discussed with reference to FIG. 1. In the vertical welding position, the molten metal pool 1 has tendency to flow downward because of the force of gravity, but the surface tension of molten metal maintains the pool 1 to some extent. The volume of the molten metal pool 1 which is maintained or sustained by the surface tension of molten metal is of course limited. In order to increase the volume of the pool 1, an additional force must be exerted to it. In the case of the vertical downward welding, when the contact angle .theta. exceeds a certain limit, to so-called cold lap occurs with the resultant poor welds. This is the reason why the vertical downward welding has a low welding rate. In FIG. 1, reference numeral 2 denotes a base metal; 3, a torch; and 4, an arc.
As described above, the low-frequency pulse TIG welding processes have an inherent disadvantage in that the weld deposition rate is low. In addition, maintainability of a molten metal pool by an arc is low. As a result, when the low-frequency pulse TIG welding processes are used in all position welding, the welding efficiency further drops as compared with other welding processes such as MIG welding processes. The high-frequency pulse TIG welding processes are superior in metal deposition rate and maintainability of a molten metal pool to the low-frequency pulse TIG welding processes, but has a disadvantage in that the applications in the field-sites are difficult as described elsewhere.
There are welding processes which may be called "intermediate-frequency pulse TIG welding" because they use the frequencies intermediate the low and high frequencies used in TIG welding processes described above, but they have not been practiced. It is well known in the art that with the welding current pulses at intermediate frequencies, high arc pressures can be obtained; but the intermediate-frequency pulse TIG welding processes have a disadvantage in that the surface of the molten metal pool immediately below the arc is depressed because of the high pressure so that the arc is surrounded with the molten metal which rises high in level. As a result, bead formation is unsatisfactory.
In view of the above, the present invention has for its object to provide a TIG welding process in which current pulses at intermediate frequencies are used to obtain high arc pressures so that welds with high qualities comparable with those obtained by the high- or low-frequency pulse TIG welding processes can be obtained and the welding efficiency or rate can be remarkably improved.
The present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.